The present invention relates to a scanning optical system used in, for example, scanning optical apparatuses such as a laser beam printer, and more particularly to a scanning optical system employing a diffractive lens surface for compensating for aberrations of refraction lenses.
Conventionally, a scanning optical system including a diffraction lens structure has been known. Examples of such a scanning optical system are disclosed, for example, in Japanese Patent Provisional Publications No. HEI 10-197820 and No. HEI 10-68903. Each of the scanning optical systems disclosed in the publications is provided with a laser source, a polygonal mirror for deflecting the laser beam emitted by the laser source, an fxcex8 lens for converging the deflected beam onto an objective surface to be scanned, such as a surface of a photoconductive drum. On one surface of the lenses included in the fxcex8 lens, a diffractive lens structure is formed.
The diffractive lens structure disclosed in Japanese Patent Provisional Publications No. HEI 10-197820 is formed with a rotationally symmetrical pattern having a plurality of concentric annular zones for compensating lateral chromatic aberration due to dispersion of the fxcex8 lens serving as a refractive lens. The diffractive lens structure disclosed in Japanese Patent Provisional Publications No. HEI 10-68903 is formed with a similar pattern for compensating shift of magnification and focal point of a plastic lens due to variation of temperature.
In the conventional scanning optical system including the diffractive lens structure, an axis connecting a front focal point and a rear focal point of the diffractive lens structure substantially coincides with the optical axis of the refractive lens structure of the fxcex8 lens. Therefore, if unnecessary components of diffracted light are generated by the diffractive lens structure, they become distributed with respect to a necessary component of the diffracted light at certain angles in a main scanning direction. It should be noted that in this specification, the term xe2x80x9cmain scanning directionxe2x80x9d is defined as a direction of movement of a beam spot on an objective surface, which is to be scanned, due to rotation of the polygonal mirror, and a term xe2x80x9cauxiliary scanning directionsxe2x80x9d is defined as a direction perpendicular to the main scanning direction on the objective surface. In the above case, therefore, beam spots formed by the unnecessary components and a beam spot formed by the necessary component are formed on the same scanning line, which extends in the main scanning direction. If an image is formed with this condition, quality of the image is deteriorated due to the unnecessary components.
For example, if a diffractive lens structure is designed such that a first order diffraction beam is used for imaging, and a second order diffraction beam and a zero-th order beam are present as unnecessary beams, beam spots formed by the second order and zero-th order beams are located spaced from, in the main scanning direction, a beam spot formed by the first order diffraction beam. Although the spots formed by the unnecessary order diffraction beams are not focused on the objective surface in the main scanning direction, they are focused in the auxiliary scanning direction. Therefore, the energy applied to a unit area is not negligible.
Further, modulation (i.e., turning ON/OFF) of the light source is controlled in accordance with a scanning position of the first order beam, and when the light source is turned ON/OFF, the unnecessary components are also turned ON/OFF. If the zero-th and/or second order diffraction light beam is located at a portion which should not be exposed to light, and the first order diffraction light beam is located at a position which is to be exposed to the light, since the zero-th and second order light beams strike the portion which should not be exposed to light that corresponds to anon-printed portion on a sheet, for example, a plurality of black dots may be printed on the portion.
Further, according to the conventional diffraction lens structure, the unnecessary diffraction components and the necessary diffraction component are incident on a point on the objective surface at different timing. However, since all the components proceeds to the point along substantially the same optical path, it is difficult to spatially eliminate only the unnecessary components.
It is therefore an object of the invention to provide an improved scanning optical system employing a diffractive lens structure for compensating aberrations of refractive lens structure, which is capable of spatially splitting unnecessary diffraction beams from a necessary diffraction beam.
For the above object, according to the present invention, there is provided a scanning optical system, which is provided with a light source that emits a light beam, a deflector that deflects the light beam emitted by the light source to scan, in a main scanning direction, within a predetermined angular range, and a scanning lens system that converges the light beam deflected by the deflector on a surface to be scanned. The scanning lens system has positive power. Further, the scanning lens system has a plurality of lens surfaces, and a diffractive lens structure is formed on at least one of the plurality of lens surfaces. The diffractive lens structure compensates for aberration in the main scanning direction caused by characteristics of a refractive lens structure of the scanning optical system. The diffractive lens structure is arranged such that an axis connecting a front focal point and a rear focal point of the diffractive lens structure is shifted, in an auxiliary scanning direction, with respect to an optical axis of the refractive lens structure of the scanning lens system.
With this structure, diffraction light components are spatially separated in the auxiliary scanning direction. Therefore, it becomes possible to selectively shield or allow a component to proceed.
Optionally, the diffractive lens structure may be formed as a part of a rotationally symmetrical pattern including a plurality of concentric annular zones.
Preferably, the diffractive lens structure is formed on a planar lens surface. Then, a mold of the diffractive lens can be manufactured easily.
Further optionally, the scanning optical system may include a light shielding member for shielding predetermined diffractive light components, the light shielding member being located between a lens provided with the diffractive lens structure and the surface to be scanned.
With this light shielding member, unnecessary diffraction light components can be shielded.
Still optionally, the scanning optical system may be provided with a prismatic diffractive lens structure for compensating for shift of a scanning line, on the surface to be scanned, in the auxiliary scanning direction due to variation of wavelength of the beam.
In a particular case, the prismatic diffractive lens structure may be provided between the light source and the deflector. Alternatively, the prismatic diffractive lens structure may be provided between the deflector and the surface to be scanned.